1. Field of the Invention
The invention relates to a device for capturing high energy radiation emitted from a radiation source within an examination object by means of a detector. Such a device is used in nuclear medicine.
2. Description of the Prior Art
In the field of nuclear medicine, which represents a technically highly developed subarea of medical engineering, radiation energies in the region of 70 kiloelectronvolts (keV), and higher are applied to perform examinations and treatments. Clinically widespread applications of nuclear medicine take the form of single photon emission computed tomography (SPECT) and positron emission tomography. The radiation used for SPECT and PET is gamma radiation, i.e. high energy electromagnetic radiation, and so is generally higher in energy than x-ray radiation. In order to generate the requisite gamma rays, SPECT uses radioactive decay processes of substances that are bound to tracers in which gamma rays are emitted and which are administered to the examination object via an oral, intravenous or respiratory route prior to the examination. With PET, the examination object is likewise administered a radioactive substance that decays while emitting positrons, i.e. by means of decay known as beta-plus decay. The positrons emitted by the radioactive substance then annihilate in the examination object with the resulting electrons producing characteristic gamma radiation.
The tracer or radioactive substance usually participates in the examination object's metabolism that is to be examined and thus becomes concentrated in certain tissue of the examination object, this tissue being deeply involved in the metabolism. A specific tracer is selected according to the purpose of the examination. For instance, various types of tumor can be identified using glucose tracers, because tumors reveal a higher energy consumption and therefore a higher glucose consumption than the surrounding tissue. The gamma rays generated directly or indirectly by means of the radioactive substances usually emerge, weakened by absorption, from the examination object. The gamma rays emerging from the patient permit a static and/or dynamic record of concentrations and where applicable depletions of gamma radiation sources in specific regions of the body, thus providing an indication of the metabolic function of the specific regions of the body, in particular organs, of the examination object.
The radioactive substances used for PET emit positrons and neutrinos, though only the positrons are of significance. If a positron strikes an electron in the environment—i.e. the examination object—the two annihilate producing two gamma quanta having an energy of 511 keV or 511 kiloelectronvolts and moving in exactly opposite directions. The exactly opposite direction of the two gamma quanta and the specific energy of the gamma quanta are based on the energy conservation law as well as on the law of conservation of momentum and angular momentum, which are also applicable for such events. The two gamma quanta are captured by means of a detector. For the purpose of the analysis, only such detection events are used in which gamma quantum detection has taken place on opposite detectors at essentially the same time. This is referred to as coincidence measurement.
A further difference between SPECT and PET is that the gamma rays required for the examination are produced differently—in SPECT they are produced directly by radioactive decay and in PET they are produced indirectly by positron-electron annihilation. In SPECT, the representation of the distribution of the radiation sources in the examination object, which can be reconstructed from the captured gamma rays, shows the distribution of the radioactive substance in the examination object, whereas in PET it yields the distribution of the annihilation sites of positrons and electrons.
To detect the gamma rays emerging from the body in PET and SPECT, at least one ray detector that can be rotated around the examination object is operated to capture the gamma rays in a specific solid angle area. Since, statistically, radiation is emitted randomly in all directions when the tracer radioactively decays, regardless of whether it is gamma radiation or positron radiation, SPECT devices as well as PET devices usually are constructed such that the radiation detectors each cover a solid angle area which is as large as possible around the area of examination of the patient. In this way it is possible to detect an as large as possible proportion of gamma radiation emerging from the examination area.
Conventional detectors used for this purpose primarily are scintillation detectors. Semiconductor detectors such as cadmium zinc telluride detectors are now increasingly used. When the gamma rays strike a scintillation crystal incorporated within a detector, for example sodium iodide (NaI) doped with thallium (TI), electron processes will be triggered by the incident gamma rays. These processes excite the luminescent centers of the scintillation crystal and the gamma rays are converted into low energy electromagnetic radiation, usually in the visible spectral range. The optical signal is then usually converted into an electrical signal, e.g. by a photodetector. The electrical signal is then amplified. The conversion and amplification are usually carried out within a common apparatus, for example by a photomultiplier.
With appropriate capturing of gamma rays, both SPECT and PET make it possible to determine a spatial representation of the distribution of the gamma radiation sources in the examination object. A further application of SPECT and PET is to examine metabolic functions in living organisms, particularly in specific organs, with cranial vascular diseases, tumors and receptor diseases such as coronary heart diseases, etc.
United States Application Publication 2004/0217292 discloses a positron emission tomography apparatus which has continuously rotating detectors. The positron emission tomography apparatus has two or more detectors that are arranged on a rotating carriage system in a removable fashion. Each detector includes an array of scintillators which is connected to a light guide. The light guide is in turn connected to an array of photodetectors, with the result that the optical signals generated by the scintiliator array are converted into electrical signals. Each detector furthermore incorporates data processing electronics for the purpose of processing the electrical signals fed thereto. Such a positron emission tomography apparatus is disadvantageous because the device is very heavy, is expensive to purchase and occupies considerable space. Furthermore, such positron emission tomography apparatuses are usually set up at a stationary operating location and can cause anxiety in patients because they have to be in an enclosed space.